1. Field of the Invention
The present invention relates to color processing for generating a color conversion table for gamut mapping.
2. Description of the Related Art
In recent years, with the prevalence of personal computers and work stations, desktop publishing (DTP) and computer-aided design (CAD) have been widely used in general. Along with this, a color reproduction technique of matching a color reproduced on a monitor with a color reproduced on a printing medium using colorants is becoming increasingly important. For example, DTP operates in a workflow of executing creation, editing, modification, and the like of a color image on a monitor and then causing a color printer to output the color image. In this workflow, a user strongly demands that the color appearance of a color image (to be referred to as a monitor image) on the monitor matches that of an output image (to be referred to as a print image) of the printer.
However, it is difficult to match the appearances of a monitor image and print image in the color reproduction technique. This difficulty arises due to the following reasons.
A color monitor presents a color image by emitting light with a particular wavelength by using, e.g., phosphors. On the other hand, a color printer presents a color image by absorbing light with a particular wavelength by a colorant such as inks and using the residual reflected light. The color gamut of the color monitor is greatly different from that of the color printer because of the difference of the image display mode. Furthermore, even in a color monitor, the color gamut of a liquid crystal monitor which displays the three primary colors by using a color filter is different from that of a CRT which displays the three primary colors by using electron guns and phosphors. Of course, the color gamut of a color printer changes due to the difference of the paper quality, composition of the colorants, and the like.
FIGS. 1A and 1B are views showing a monitor gamut (FIG. 1A) and a printer gamut (FIG. 1B) as an example of a color gamut. As shown in FIGS. 1A and 1B, a color gamut greatly differs in each device. Therefore, it is impossible to completely match the colors of a monitor image and printer image, those of printer images output by a plurality of types of devices, or those of printer images output on a plurality of kinds of papers, in the calorimetric meaning.
As a technique of matching the appearances of color images by absorbing the difference of the color gamut between output media with different color gamuts, a gamut mapping technique of mapping a given color gamut onto another color gamut in a Lab space or JCH space is available. An example of the gamut mapping technique is a method of compressing and mapping a wide color gamut onto a narrow color gamut by using, e.g., a point on the lightness axis as a convergence point. According to such a mapping method, near the gamut boundary, color change is steep and image deterioration such as pseudo contour may occur. For example, FIG. 1A shows a monitor gamut and tone characteristics of the cyan hue. When performing gamut mapping, the tone characteristics may become distorted as represented by the solid lines in FIG. 1B. Particularly, in a region indicated by the circle in FIG. 1B, color change is steep and tone characteristics contort. Note that the dotted line in FIG. 1B represents a printer gamut.
In addition, as an element technique of a color matching technique, a color interconversion technique of a device independent color and device color is important. Color conversion from a device independent color into a device color in the color interconversion technique is used to accurately reproduce the mapping result by an actual device (e.g., a printer). In order to attain satisfactory tone reproduction, image deterioration such as an unnatural tone jump by this color conversion must be prevented as much as possible. However, this color conversion results in an optimization problem and often requires technically complex processing, e.g., use of a nonlinear programming to implement the conversion. It is difficult to keep satisfactory tone character in any case.
This problem is translated into a technical problem of how to correct the color conversion with poor tone reproduction. That is, a color interconversion technique which solves this technical problem and attains satisfactory tone reproduction is demanded. It is also demanded to achieve satisfactory color reproduction in various color conversions such as color conversion by mapping, color conversion from a device independent color into a device color, and the like.